Aircraft: A320
Phase of flight: Taxi
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown

After pushback and during taxi; ground control advised us that we had a 1:10 minute ground delay for our flight due to construction at [destination airport]. Ground control gave us a place to pull over and we shut down both engines and left the APU running. After finishing checklists and discussing the delay and our ample fuel status; the CA asked if I was okay with opening the cockpit door. I replied 'absolutely.' So; as I was communicating our status with dispatch via ACARS; several passengers wanted to look in the cockpit. The Captain and I agreed that this was a fine idea as we thought it would be great for passengers to have something of interest to enjoy while we were waiting on the ramp for an hour or so. Several adults and children came up to the cockpit to take a look around and ask questions with some folks taking pictures; etc. It was really enjoyable handing out wings to the kids and talking to parents about [our company] and aviation in general. About 30 minutes passed and ground control advised us that our delay had been shortened and asked us to get ready for departure. We had all passengers sit down; secured the cockpit door and performed our preflight checklists as per SOP.We took off and landed without incident. The flight itself was uneventful.The reason I am reporting this is; during the cruise phase of the flight the Captain and I talked about the recent [industry problems] and wondered if any changes had been made to SOP concerning photos; etc. I looked in the FOM for anything concerning cockpit access on the ground and found the last sentence of a paragraph; which states that you should not allow cockpit access after you have pushed back; even during a ground delay with engines shut down; which was our exact situation. It actually states that you should not conduct cockpit tours; etc. Which is pretty much exactly what we did. So we talked about that and decided we had made a mistake and should report it. We both talked about learning a new lesson in the FOM. I've had people up in the cockpit many times at the gate; so I just deferred to that way of thinking. It was quite enjoyable interacting with our passengers but I realize now that I should have referenced the FOM first. On this flight; as we approached the runway ATC informed us we had a wheels up time of 1 hour 7 minutes later. We were directed where to stop and shut down the engines and we did so.During my years as a First Officer (FO); many times during similar circumstances; both on and off the gate; Captains have opened the door and invited customers to visit the flight deck while waiting for the wheels up time. It was my understanding that with both engines off; opening the door was authorized. This time I thought it a good gesture for the passengers so I asked the FO if he was ok with doing that and he agreed. At the appropriate time we closed the door; prepared for flight and proceeded without incident. During cruise the FO and I began to wonder if the FOM contained specific guidance about such activity; and upon inspection we found in FOM that it is not authorized. I don't know if the guidance changed in recent years or if it has always been there; but I acted based on prior experience without first verifying if it was permitted by the FOM. A320 flight crew reported receiving a ground delay after taxi and elected to shut down and allow passengers to visit the cockpit. Once airborne the crew decided to check the FOM and found that the practice was no longer allowed.